Geogenic and atmospheric sources for volatile organic compounds in fumarolic emissions from Mt. Etna and Vulcano Island (Sicily, Italy)

In this paper, fluid source(s) and processes controlling the chemical composition of VOCs (Volatile Organic Compounds) in gas discharges from Mt. Etna and Vulcano Island (Sicily) were investigated. The main composition of the Etnean and Volcano gas discharges is produced by mixing, at various degree, of “magmatic” and “hydrothermal” components. VOC composition is dominated by alkanes, alkenes and aromatics, with minor, though significant, concentrations of O-, S- and Cl(F)-substituted compounds. The main mechanism for the production of alkanes is likely related to pyrolysis of organic matter-bearing sediments that interact with the ascending magmatic fluids. Alkanes are then converted to alkene and aromatic compounds via catalytic reactions (dehydrogenation and dehydroaromatization, respectively). Nevertheless, an abiogenic origin for the light hydrocarbons cannot be ruled out. Oxidative processes of hydrocarbons at relatively high temperatures and oxidizing conditions, typical of these volcanic-hydrothermal fluids, may explain production of alcohols, esters, aldehydes, as well as O- and S-bearing heterocycles. By comparing the concentrations of HCFCs (hydrogenated chlorofluorocarbons) in the fumarolic discharges with respect to those of background air, it is possible to highlight that they have a geogenic origin likely due to halogenation of both methane and alkenes. Finally, CFC (chlorofluorocarbon) abundances appears to be consistent with background air, although the strong air contamination that affects the Mount Etna fumaroles may mask a possible geogenic contribution for these compounds. On the other hand, the Vulcano gases have a low air contribution that prevents the detection of geogenic CFCs.